1. Field of the Invention
This invention relates to a method of controlling a floating zone applied to a crystal manufacturing system based on the FZ (floating zone) method.
2. Description of the Prior Art
In floating zone control, it is particularly important to control the diameter of the crystal rod at the crystallization boundary. If the response performance and the stability of control of the crystal diameter at the crystallization boundary are inadequate, disturbance of crystal growth takes place and makes the resulting product defective. Even if the formation of any defective products can be avoided, the length of a conical portion which cannot be salable as the product may become excessively large, and irregularities in the surface of the major cylindrical portion of the crystal rod in the axial direction and, hence, the depth to which the surface of the cylindrical portion is ground to eliminate such irregularities may be increased, resulting in an increase in the manufacturing loss.
Where only the crystal diameter at the crystallization boundary is controlled, an unmelted conical portion in the floating zone projecting from the lower end surface of the raw-material rod becomes closer to the upper end surface of the growing crystal rod if the axial length of the floating zone becomes very short. In this state, the upper end surface of the crystal rod is decreased in temperature at its center relative to that of its peripheral portion and thereby crystallographic disturbances often occur, and furthermore, there is a possibility of the lower end of the unmelted conical portion becoming fixed to the upper end surface of the crystal rod, which makes it impossible to continue to grow the crystal. If the axial length of the floating zone becomes excessively large, the floating zone may be broken by spilling over.
If the electric power supplied to the induction heating coil is changed, both the crystal diameter at the crystallization boundary and the axial length of the floating zone are changed. Also, these quantities are changed in response to a change in the speed at which the raw-material rod is moved downward relative to the induction heating coil.
As described above, the fundamental subject of the conventional floating zone control method is to optimize the control of the above-mentioned power supply and the above-mentioned relative speed so as to enable the crystal diameter at the crystallization boundary and the axial length of the floating zone or other similar quantities to become approximated to desired values stably and promptly. Many attempts have been made to improve the method in this respect.